Learning Forum

[Maria Perez]

I keep getting an error with the following expression for shear stress. I need to get sqrt(x) but only for the positive values (x>0). Can someone please help me? 

[NickFL]

There is a couple of things I notice here. Do you see how the ( is italicized before the [ ? That is letting you know it is treating the ( as part of the units. The easiest solution would be to add a * 1 before each of the units. This will multiply you scalar by a unit value of each. The alternative would be to put the units inside the (). I would also double check your order of operations. Do you want (15^3)/2 or do you want 15^1.5? I would assume 15 [m/s] ^ 1.5 to obtain units of pressure.

[Maria Perez]

That's not giving me an error, what gives me an error is the sqrt(X) 

[NickFL]

The sqrt root is giving you an error because the () are not closed on the function.

[Maria Perez]

 

 

It is closed, if I change X for 1[m] it works and gives me Pa as a result which is what I need. So the expression is not the problem. The problem is that the sqrt is taking negative values of X and that’s why it gives me an error. That’s why I specifically asked how to make the expression take only positive values of X. 

 

 

[NickFL]

X is based on the geometry. So if you have cells in your domain that have a negative X, then you will have problems. Basically in DesignModeler or SpaceClaim you created the domain with negative values of X. Without going back into these programs and having to remesh, the simple solution would be to scale it by adding whatever the min(X) to your X.

Example, if you have a domain that has X values that go to -5 [ m ], change the X to ( X + 5 [m] ).

[Rob]

By Scale, you mean Transform: that'll allow you to shift the domain. I would question linking shear to domain position, but that's for your model. 

[Maria Perez]

Gotcha, I will try that. Thank you so much